1. Field of the Prior Art
This invention relates to an electromagnetic valve and more particularly to an electromagnetic valve for linearly controlling the flow quantity between the inlet port and the exhaust port in response to an electric current applied to the electromagnetic mechanism from an electric control circuit.
2. Description of the Prior Art
A typical electromagnetic valve of this type is disclosed, for example, in U.S. Pat. No. 4,493,474. This typical electromagnetic valve includes a valve housing having a first chamber opened an inlet port and a second chamber opened an exhaust port, a shaft member movable disposed toward to an axial direction in response to an electric current applied the electromagnetic mechanism from an electric control circuit in a motor housing and the valve housing, a valve member movable disposed so as to open and close the communication between the inlet port and the exhaust port with the movement of the shaft member. A pressure balance mechanism comprising a bellows member is disposed in a concentric configuration to the shaft member in both housings. The bellows member is airtightly fixed at its both end portions between both housings and the valve member, respectively and forms an airtight chamber. The airtight chamber is communicated with the second chamber through a passage means formed in the valve member.
In this prior art, when the valve member is closed by a spring load, since the pressure in the second chamber equalizes to the pressure in the bellows member, the pressure difference between both chambers does not operate the valve member so as to urge the valve member toward to the opening position or the closing position. Therefore, in this electromagnetic valve, it is not necessary to generate a large linear attraction force in the electrtomagnetic mechanism and it is possible to be minimized the size of the electromagnetic mechanism.
In the above typical electromagnetic valve, however, since the pressure balance mechanism comprises a bellows member, it is necessary to increase the size of a bellows portion of the bellows member in order to be minimize the force for moving the bellows member in the axial direction. As a result, the size of the electromagnetic valve becomes large in the axial direction. Furthermore, since it is necessary to minimize the thickness of the bellows member, a special and expensive material, for example, such as Teflon, is necessary as the material fo the bellows member. As a result, the electromagnetic valve becomes expensive. Furthermore, since the valve member is fixed to the one end of the shaft member, it is not able to ensure the airtightness of the valve member. Therefore, it is necessary to improve the accuracy of the finished size of a valve seat portion of the valve housing and the valve member, so that there is a drawback that the cost for manufacturing the electromagnetic valve becomes expensive.